Modern fixed wing commercial transport aircraft share features in common with their predecessors, including wings, a fuselage, control surfaces and engines. Continuous advancement in aerodynamics, materials, engine power and efficiency, and component design contribute to faster, safer air travel. The arrangement of the common aircraft features, however remains unchanged and the cylindrical fuselage has remained a recognizable and consistent feature of commercial aircraft.
Particularly for commercial transport aircraft, the fuselage is typically divided into separate volumes. In many instances, passengers sit in a volume referred to as the passenger cabin. The passenger cabin is often separated from volumes below in which cargo is carried, in which airplane mechanical and electrical systems are located, and through which air flows. The cargo volumes may be separated axially by the wing box and main landing gear bay into the forward and aft cargo compartments. The combination of the passenger cabin and crown volumes may be referred to collectively as the upper lobe and the combination of the cargo compartments, bilge, left and right cheeks, and floor beam volumes may be referred to collectively as the lower lobe. In many situations, an uncontrolled air flow exists between the upper and lower lobes.
Conditioned air is provided to the passenger volume to pressurize the airplane fuselage, control temperature, contaminants and odors. This air must flow from the upper lobe to the lower lobe of the fuselage where it can either be recirculated back to the passenger cabin or be released to the ambient atmosphere from which it was originally drawn. Control of air flow from the upper to lower lobes is intended to occur through return air grilles located near the interface of the floor and passenger cabin sidewalls. However, a significant portion of the air flow supplied to the passenger cabin returns to the lower lobe through a path between the airplane skin and passenger cabin sidewall, through gaps in sidewall insulation blankets and fire stops, i.e. the air flow is uncontrolled. This sidewall air flow is contained in a channel that is bounded outboard by the airplane fuselage skin, inboard by the passenger cabin sidewall panel, and axially by the fuselage frame channels. This region may be referred to as the aircraft sidewall volume. In many situations, heat is transferred between the air and the bounding surfaces by convection, and from the bounding surfaces to adjacent structure by conduction as the air moves through the sidewall volume from upper lobe to lower lobe.